1. Field of the Invention
This invention relates to compositions, devices, and methods for determining the presence of a ligand in a liquid medium based on the affinity of the ligand for a specific binding partner thereof. In particular, this invention relates to compositions, devices, and methods for use in specific binding assays which do not require a separation step and which do not employ radioactive materials or modified enzymes as the labeling substance.
The desirability of a convenient, reliable, and non-hazardous means for detecting the presence of low concentrations of substances in liquids is self-evident. This is particularly true in the field of clinical chemistry where constituents of body fluids which may appear in concentrations as low as 10.sup.-11 molar are known to be of pathological significance. The difficulty of detecting such low concentrations is compounded in the field of clinical chemistry where sample size is usually quite limited.
Classically, substances have been detected in liquids based on a reaction scheme wherein the substance to be detected is a necessary reactant. The presence of unknown is indicated by the appearance of a reaction product or the disappearance of a known reactant. In certain instances, such an assay method may be quantitative, based on a measurement of either the rate of appearance of product or disappearance of reactant or measurement of the aggregate amount of product produced or reactant consumed in attaining equilibrium. Each assay reaction system is necessarily either limited to use in the detection of only a small group of substances or is non-specific.
The search for assay systems which are highly specific yet adaptable to the detection of a wide range of substances has evolved the radioimmunoassay. In this system a known amount of a radiolabeled form of the substance to be detected is allowed to compete with the unknown for a limited quantity of antibody specific for the unknown. The amount of the labeled form that becomes bound to antibody varies inversely with the level of unknown present. Inherent in the radioimmunoassay technique is the need to separate the labeled form of substance to be detected which becomes bound to antibody from that which does not become so bound. While various ways of accomplishing the required separation have been developed, as exemplified in U.S. Pat. Nos. 3,505,019; 3,555,143; 3,646,346; 3,720,760; and 3,793,445, all require at least one separate manipulative step, such as filtering, centrifuging, or washing, to insure efficient separation of the bound-labeled form from the unbound-labeled form. The elimination of the separation step would greatly simplify the assay and render it more useful to the clinical laboratory.
The use of radioactive materials in immunoassays has been eliminated to some degree by the use of enzyme-tagged materials in place of radiolabels. As exemplified by U.S. Pat. Nos. 3,654,090 and 3,791,932, the manipulative steps necessary for carrying out the enzyme-tagged immunoassays are for the most part the same as those required in radioimmunoassays and include the cumbersome separation step. An additional disadvantage of using enzyme-tagged materials is that each enzyme used as a tag must be individually chemically modified for use in the formation of the tagged conjugate. The use of other tagging materials has been suggested, such as the use of coenzymes or viruses, Nature 219: 186 (1968) and the use of fluorescent-labels, French Pat. No. 2,217,350, corresponding to U.S. Pat. No. 3,880,934.
2. Description of the Prior Art
While the radiolabeled and enzyme-tagged immunoassays may undergo future improvement in terms of expansion of the range of substances detectable thereby or of simplification of the procedure, by their nature they will always require some type of separation step. Recently, a different approach was disclosed which does not require a separation step and therefore has been referred to as a homogeneous system, in contrast to a heterogeneous system in which separation is essential. U.S. Pat. No. 3,817,837 discloses a competitive binding assay method involving the steps of combining the liquid to be assayed with a soluble complex consisting of an enzyme as a labeling substance covalently bound to the ligand to be detected and with a soluble receptor, usually an antibody, for the ligand; and analyzing for the effect of the liquid to be assayed on the enzymatic activity of the enzyme in the complex.
While this method has the advantage of not requiring a separation step because reaction between the enzyme-bound-ligand complex and the receptor results in inhibition of the enzymatic activity of the enzyme in the complex, the method nonetheless is severely restricted in its ability to be adapted to widely varied assay requirements. For instance, it is clearly essential that in the fabrication of the enzyme-bound-ligand complex, the substance or ligand to be detected must be coupled to the enzyme in a carefully controlled manner so that the coupling site is close to the enzymatically active site on the enzyme. This is required in order that upon reaction between the complexed ligand and the receptor, the enzymatically active site is blocked. Enzymes vary greatly in their size, ranging in molecular weight from about 10,000 to 1,000,000. Thus, for a receptor in the form of an antibody having a molecular weight of between 150,000 and 300,000 to be capable of physically blocking the active site on an average enzyme of 500,000 molecular weight or greater, the coupling site must be precisely controlled. Due to the complex chemical structure of enzymes, precise control of such chemical linkage is indeed difficult, and one would expect that even upon screening a wide variety of enzymes only a small number would be found to be of use in this homogeneous assay system.
Moreover, it is critical for the purpose of obtaining quantitative test results to precisely control the ratio of the number of enzymes to the number of ligands in each enzyme-bound-ligand complex. Here also, the complex peptide structure of enzymes makes such control difficult. It would again be expected that only a small number of enzymes would have suitable molecular structure to ensure necessary control of the ligand/enzyme ratio.
The prior art homogeneous assay method is stated to involve an enzyme amplification and thus to be highly sensitive. However, since the labeling substance, namely the enzyme, is itself the limiting factor determining the sensitivity of the prior art assay method, the versatility of the method is severely restricted. The sensitivity is clearly limited to the catalytic activity of the particular enzyme in the enzyme-bound-ligand conjugate. The versatility of the prior art method is therefore restricted not only by the coupling requirements for formation of a useful conjugate but also by the dependence of the sensitivity of the assay that employs such conjugate on the activity of the particular conjugated enzyme.
An additional disadvantage of the prior art homogeneous assay method arises in its application to the testing of biological fluids such as urine and serum. It is to be expected that significant amounts of the enzyme species comprised in the enzyme-bound-ligand conjugate may appear in the fluid sample to be tested thereby creating an uncontrollable background activity which would severely affect the accuracy of the assay method. Therefore, in order to form an assay system that is useable in testing biological fluids of humans or animals, exotic enzymes not endogenous to such fluids must be selected for use in forming the enzyme-bound-ligand conjugate with the result that the versatility of the assay method is even further restricted.
It is therefore an object of the present invention to provide a novel test composition, device, and method for detecting a ligand in a liquid which do not require a separation step and which do not employ inconvenient radioactive materials or modified enzymes as the labeling substance.
Further, it is an object of the present invention to provide a homogeneous specific binding assay method and system which are more versatile and convenient than those of the prior art.
Another object of the present invention is to provide a homogeneous specific binding assay method and system which employ a labeling substance which is capable of being coupled to the ligand or to a specific binding partner thereof more conveniently than can the enzyme of the prior art method.
A further object of the present invention is to provide a homogeneous specific binding assay method and system which employ a conjugate comprising a labeling substance whose activity is more readily affected by a specific binding reaction than is the enzyme of the prior art method.
It is also an object of the present invention to provide a homogeneous specific binding assay method and system which employ a conjugate comprising a labeling substance any change in the activity of which is more conveniently detectable using a wide variety of sensitive reaction systems than is any change in the activity of the enzyme in the prior art method.
It is a further object of the present invention to provide a homogeneous specific assay method and system which are more readily applicable to the testing of biological fluids than those of the prior art.